While smoking prevalence has declined for both men and women over the last two decades, rates among women have shown a much shallower decrease and, in recent years, prevalence of cigarette initiation has been higher for girls than boys. Smoking among women of child-bearing age has significant negative health consequences for mother and child, increasing fetal and infant morbidity and mortality. Women are both less likely to initiate a quit attempt and more likely to relapse if they do quit. Nicotine replacement therapy (NRT), still the most widely used smoking treatment intervention in the United States, is less effective for women compared with men, and women report less craving reduction on NRT. The endogenous opioid system is involved in smoking initiation, nicotine craving and reward as well as nicotine withdrawal symptoms. Interestingly, research suggests that sexual dimorphic features of the endogenous mu-opioid system may in part explain gender differences in nicotine effects. To better understand the role of the mu-opioid system in poorer NRT responses in women, this proposal will examine NRT effects on mu opioid receptor binding potential (MOR BP) in female compared to male smokers during active versus placebo NRT. Specifically, nicotine dependent women and men in active smoking status will undergo PET imaging for MOR BP measurement using 11C-carfentanil. Following baseline PET measurement in active smoking (scan 1), smokers will be randomized to active or placebo nicotine replacement therapy ((A-NRT or P-NRT); 72 hours later, a second scan will be obtained. As a reference group, demographically-matched women and men who have never smoked will undergo two scans as well. Behavioral measurements of nicotine reward, craving and withdrawal will be obtained repeatedly across the protocol. The proposed research will provide significant new, gender-specific information of scientific and clinical relevance on the function of the mu-opioid system in nicotine dependence and therapeutic effectiveness of nicotine replacement therapy.